Biological decolorization of dye solution containing malachite green by Pandoraea pulmonicola YC32 using a batch and continuous system

Abstract

In our study, we have isolated a relatively newly identified bacteria species, Pandoraea pulmonicola YC32, and first assessed its capability to treat malachite green (MG). The effects of various factors on decolorization efficiency were investigated in a batch system. The decolorization efficiency was found to be optimal within a pH of 7–10 and it increased, with increasing initial MG concentration up to 100 mg/l. The relationship between the decolorization rate and MG concentration agreed with Lineweaver–Burk equation. The apparent kinetic parameters, R_MG,max and K_m, were 6.23 mg-MG/g-cell/h and 153.4 mg/l, respectively. The initial step in the biodegradation pathway of MG by P. pulmonicola YC32 was a reduction or N-demethylation reaction. We achieved a decolorization efficiency of 85.2% with 50 mg/l MG in the immobilized P. pulmonicola YC32 continuous column system. This is the first report on the application of a continuous column system to decolorize MG using a microorganism.

Introduction

Malachite green (MG) is a triphenylmethane dye that is used extensively in the textile and fish farming industries as a biocide. It is toxic to human beings, affecting both the immune and reproductive system [1], and has a highly toxic effect on freshwater fish following either acute or chronic exposure [2]. Consequently, any discharge of an MG-containing solution into a river or stream will adversely affect on the exposed aquatic organisms. Although, the use of MG for controlling fungal infections and ectoparasites in the aquaculture industry is prohibited in the USA because of its carcinogenic nature [3], it is still used in some areas of the world because of its low cost [4]. As there is a significant health risk to humans who eat fish contaminated with MG, it is very important to establish a method to remove this substance from water/solutions.

A number of studies have been carried out in which physico-chemical methods, such as adsorption, precipitation, photodegradation, osmosis, and membrane filtration, have been used to treat MG. However, such methods have proved to be methodologically demanding, relatively inefficient, and time-consuming [5], [6], [7]. Focus has therefore turned to biological processes as a viable alternative as such systems are cost-effective and environmentally friendly, and they produce less sludge [8], [9], [10]. However, there was no MG degradation pathway ever discussed in these studies.

A variety of organisms have been identified as being capable of decolorizing and degrading MG, including a microalgae (Cosmarium sp.) [11], yeast (Saccharomyces cerevisiae) [4], and fungus (Ischnoderma resinosum) [12]. The use of the bacteria Kurthia sp. and Kocuria rosea to treat MG were reported as well [13], [14]. However, few studies have been made on the use of bacteria to treat MG in a “continuous” system.

Pandoraea pulmonicola was first identified by Coenye et al. [15]. The cells of P. pulmonicola are Gram-negative, non-sporulating. They are motile by means of a single polar flagellum. Catalase activity is present and P. pulmonicola can assimilate caprate and dl-lactate. Growth is observed at 30 and 37 °C. Although there are no published data currently available on its efficiency to biodegrade MG, preliminary experiments carried out in our laboratory revealed that this specie or strain has a high capability to degrade MG. In addition, P. pulmonicola can be cultured easily in culture medium.

The aim of the study reported here was to isolate MG-degrading microorganisms from contaminated soil nearby a textile plant, determine the basic physiological characteristic of the isolated strain, and provide further details on P. pulmonicola YC32 in terms of its capability to degrade MG. The effect of various operating parameters (initial concentration of MG, pH, and cell numbers of P. pulmonicola YC32) on MG removal in a batch system was evaluated, and the primary metabolic pathway during the biodegradation process was established. The removal characteristics of the immobilized P. pulmonicola YC32 to degrade MG in a continuous system were also investigated.